It will take at least as long as it will take for you to realize that this question is unanswerable. A mature Christian is one who exercises godly wisdom and does not become disillusioned by the things that people do, but, rather, stays focused on Christ.

A mature Christian is one who realizes the vast amounts that he does not yet know. Many people develop spiritual pride and think they know more than the average Christ-follower. These people are not mature! This old saying is very true in Christianity: The more you know, the more you know you don't know!

A Christian who realizes the truth about his or her own knowledge, who seeks the counsel of the Holy Spirit before jumping into action and consults the Bible for Scriptural truths is most likely a mature Christian.

Becoming a mature Christian can be easy or it can be very difficult. How willing are you to surrender yourself, fully, to the molding process of the Holy Spirit? Maturing, refining, shaping, and molding can be painful processes. But, always, the end result is more beautiful than could ever be imagined. The Holy Spirit does the renewal work:

Colossians 1.21-23: "And you, that were sometime alienated and enemies in your mind by wicked works, yet now hath He reconciled. In the body of His flesh through death, to present you holy and unblameable and unreproveable in His sight: If ye continue in the faith grounded and settled, and be not moved away from the hope of the gospel, which ye have heard, and which was preached to every creature which is under heaven; whereof I Paul am made a minister."

Philippians 1.6: "Being confident of this very thing, that He which hath begun a good work in you will perform it until the day of Jesus Christ."
Maturity comes through listening to the Word in a Bible believing fellowship:

Colossians 1.28: [Christ,] Whom we preach, warning every man, and teaching every man in all wisdom; that we may present every man perfect in Christ Jesus.

Maturity through circumstances:

James 1.3-5: "Knowing this, that the trying of your faith worketh patience. But let patience have her perfect work, that ye may be perfect and entire, wanting nothing. If any of you lack wisdom, let him ask of God, that giveth to all men liberally, and upbraideth not; and it shall be given him."

Increased maturity even through enduring difficult circumstances without complaining:

Philippians 2.14-16: "Do all things without murmurings and disputings: That ye may be blameless and harmless, the sons of God, without rebuke, in the midst of a crooked and perverse nation, among whom ye shine as lights in the world. I know both how to be abased, and I know how to abound: everywhere and in all things I am instructed both to be full and to be hungry, both to abound and to suffer need. I can do all things through Christ which strengthened me."

We are called to participate in helping Christ bring ourselves, and others into spiritual maturity:

Ephesians 4.11: "And He gave some, apostles; and some, prophets; and some, evangelists; and some, pastors and teachers; for the perfecting of the saints, for the work of the ministry, for the edifying of the body of Christ: Till we all come in the unity of the faith and of the knowledge of the Son of God, unto a perfect man, unto the measure of the stature of the fullness of Christ: That we henceforth be no more children, tossed to and fro, and carried about with every wind of doctrine, by the sleight of men, and cunning craftiness, whereby they lie in wait to deceive."